Taraknath Pal

Arsenic Pollution in Groundwater of West Bengal, India - An Insight into the Problem by Subsurface Sediment Analysis

Abstract Arsenic values in groundwater above the maximum permissible limit of 0.05 mg/l have been reported from several areas of the lower delta region of the Ganga-Padma river system. It is confined to areas east of Hooghly River and to the shallow aquifer ( Arsenic values in sub-surface sediments from arsenious water zones are significantly higher than values from safe water zones. The major part of the sediments, consisting of non-magnetic and some feebly magnetic minerals, is arsenic free. Some iron-rich clastic minerals, like illite, biotite, chlorite, Fe-coated grains, and also the authigenic siderite concretions, which together constitute only a small part of the sediments, are carriers of arsenic and contribute the element to contaminate groundwater. Illite, biotite and chlorite degenerate to produce iron oxide/hydroxide coating on their surface and also on the surface of some other sand particles. Arsenic is adsorbed on the iron hydroxide coating creating one kind of sink for the element. Bacteria induced growth of siderite concretions grow initially on these clastic minerals and after attaining bigger size fall off as individual globules. Arsenic gets adsorbed on these concretions forming a second kind of sink for the element. Arsenic carrying river water inundates the lower delta at the time of seasonal flood. Arsenic percolates with the water downwards to the shallow aquifer. It gets fixed in the aquifer sediments in the two ways described and is retained in the traps thus formed. Locally developed reducing condition causes dissolution of iron hydroxide coating on the surface of clastic arsenic traps and also of the siderite concretions. This leads to increase in arsenic level in groundwater at these sites. A currently active process of creation and periodic enrichment of the arsenic traps followed by their subsequent depletion through desorption and dissolution is suggested to be the reason for arsenic contamination of groundwater in this part of the delta.

Diversity, metabolic properties and arsenic mobilization potential of indigenous bacteria in arsenic contaminated groundwater of West Bengal, India.

Arsenic (As) mobilization in alluvial aquifers is caused by a complex interplay of hydro-geo-microbiological activities. Nevertheless, diversity and biogeochemical significance of indigenous bacteria in Bengal Delta Plain are not well documented. We have deciphered bacterial community compositions and metabolic properties in As contaminated groundwater of West Bengal to define their role in As mobilization. Groundwater samples showed characteristic high As, low organic carbon and reducing property. Culture-independent and -dependent analyses revealed presence of diverse, yet near consistent community composition mostly represented by genera Pseudomonas, Flavobacterium, Brevundimonas, Polaromonas, Rhodococcus, Methyloversatilis and Methylotenera. Along with As-resistance and -reductase activities, abilities to metabolize a wide range carbon substrates including long chain and polyaromatic hydrocarbons and HCO3, As3+ as electron donor and As5+/Fe3+ as terminal electron acceptor during anaerobic growth were frequently observed within the cultivable bacteria. Genes encoding cytosolic As5+ reductase (arsC) and As3+ efflux/transporter [arsB and acr3(2)] were found to be more abundant than the dissimilatory As5+ reductase gene arrA. The observed metabolic characteristics showed a good agreement with the same derived from phylogenetic lineages of constituent populations. Selected bacterial strains incubated anaerobically over 300 days using natural orange sand of Pleistocene aquifer showed release of soluble As mostly as As3+ along with several other elements (Al, Fe, Mn, K, etc.). Together with the production of oxalic acid within the biotic microcosms, change in sediment composition and mineralogy indicated dissolution of orange sand coupled with As/Fe reduction. Presence of arsC gene, As5+ reductase activity and oxalic acid production by the bacteria were found to be closely related to their ability to mobilize sediment bound As. Overall observations suggest that indigenous bacteria in oligotrophic groundwater possess adequate catabolic ability to mobilize As by a cascade of reactions, mostly linked to bacterial necessity for essential nutrients and detoxification.

Arsenic biotransformation and release by bacteria indigenous to arsenic contaminated groundwater

Arsenic (As) biotransformation and release by indigenous bacteria from As rich groundwater was investigated. Metabolic landscape of 173 bacterial isolates indicated broad catabolic repertoire including abundance of As(5+) reductase activity and abilities in utilizing wide ranges of organic and inorganic respiratory substrates. Abundance of As homeostasis genes and utilization of hydrocarbon as carbon/electron donor and As(5+) as electron acceptor were noted within the isolates. Sediment microcosm study (for 300 days) showed a pivotal role of metal reducing facultative anaerobic bacteria in toxic As(3+) release in aqueous phase. Inhabitant bacteria catalyze As transformation and facilitate its release through a cascade of reactions including mineral bioweathering and As(5+) and/or Fe(3+) reduction activities. Compared to anaerobic incubation with As(5+) reducing strains, oxic state and/or incubation with As(3+) oxidizing bacteria resulted in reduced As release, thus indicating a strong role of such condition or biocatalytic mechanism in controlling in situ As contamination.